The present invention generally relates to generating wireless communications, and particularly relates to generating channel quality information.
Channel quality information can be expressed in a number of ways, such as a channel quality indicator identifying the effective signal quality of a wireless communication receiver. The effective signal quality represents the benefits of any interference cancellation gains, diversity combining gains, coding gains, etc., that may be provided by the receiver. In other words, the output signal quality of a given receiver may be significantly better than the receiver's input signal quality, which generally is expressed as the “raw,” uncompensated ratio of received signal energy to background noise and interference.
Those skilled in the wireless communications art will appreciate the value of generating channel quality information based on the output signal quality rather than the input signal quality, because the output signal quality better represents the actual signal quality bearing on the quality of demodulation and decoding performance. That is, assuming that the channel quality information is used for link adaptation, e.g., picking the modulation and coding scheme (MCS) that is appropriate for transmitting to the receiver, using input signal quality rather than output signal quality would result in chronically underutilizing the transmission link.
Accurately determining output signal quality as part of ongoing receiver operations is not, however, a straightforward proposition for some types of receivers. In particular, non-linear receivers generally have a complex relationship between input signal quality and output signal quality. Examples of non-linear receivers include multi-stage receivers employing successive interference cancellation, and joint-detection receivers.
Further, even assuming the calculability of output signal quality, such calculations may be inaccurate under some circumstances. For example, the performance achievable by a given receiver depends on a number of variables, including propagation channel conditions. Some channel conditions translate into better receiver performance than others. Thus, even for the same receiver input signal, the receiver output signal quality will vary as a function of changing channel conditions. If channel conditions are changing rapidly, or the input signal is too weak to make reliable channel estimates, reporting output signal quality, or some metric relating to output signal quality, may lead to control errors.
For example, a supporting base station may perform transmit link adaptation responsive to receiving channel quality information from a wireless communication device, that identifies the quality of the channel in terms of some metric, such as a channel quality indicator, a rate selection indicator, or the like. Thus, the base station can adjust the transmit data rate, for example, in accordance with the channel quality indicators reported by the device. This control mechanism works well if the actual channel conditions change slowly in comparison to the link adaptation control lag. However, performing link adaptation in consideration of actual propagation channel conditions becomes unreliable if those conditions change rapidly in comparison to the link adaptation control timing.